The aim of this study was to develop an in vitro system in which an isolated bronchus from human lung was exposed, during 30 min, to a constant flow of either air or nitrogen dioxide (NO2), and to examine subsequently the contractile response of airway smooth muscle rings to carbachol, histamine, and substance P. Two proximal bronchi were mounted in an organ bath, perfused externally with Krebs-Henseleit solution and ventilated with clean air, 1.0 or 2.0 ppm NO2. The exposed bronchi were then cut into rings and mounted in a computerized organ bath system. Contractile responses to agonists were measured isometrically. In each ring, a cumulative concentration response curve was obtained to the desired agonist. We found that in vitro exposure of human lumen bronchus to a constant flow of air did not alter the contractility of the smooth muscle. Whereas in vitro exposure of the bronchus to 1.0 ppm NO2 did not significantly increase the efficacy or the potency of carbachol, exposure to 2.0 ppm NO2 increased airway smooth muscle contractions in response to carbachol, histamine, and substance P. These results indicate that our experimental preparation is well suited to study the respiratory toxicity of inhaled pollutants in order to understand further the mechanisms underlying toxicant-induced airway hyperresponsiveness.